Performance and Processability of Organic Field Effect Transistors
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Performance and processability of organic field effect transistors Milan Alt Faculty for Electrical Engineering and Information Technology Karlsruhe Institute of Technology (KIT) This document is licensed under the Creative Commons Attribution – Share Alike 3.0 DE License (CC BY-SA 3.0 DE): http://creativecommons.org/licenses/by-sa/3.0/de/ Performance and processability of organic field effect transistors Zur Erlangung des akademischen Grades eines DOKTOR-INGENIEURS an der Fakultät für Elektrotechnik und Informationstechnik des Karlsruher Instituts für Technologie (KIT) genehmigte Dissertation von Dipl.-Phys. Milan Alt geb. in Wiesbaden Tag der mündlichen Prüfung: 15. Juli 2015 Hauptreferent: Prof. Dr. Uli Lemmer Korreferent: Prof. Dr. Wolfgang Kowalsky Milan Alt: Performance and processability of organic field effect transistors © July 2015 supervisors: Uli Lemmer Wolfgang Kowalsky location: Karlsruhe time frame: July 2015 license: This document is licensed under the Creative Commons Attribution Share Alike 3.0 DE License (CC BY-SA 3.0 DE): http://creativecommons.org/licenses/by-sa/3.0/de/ The fundamental laws necessary for the mathematical treatment of a large part of physics and the whole of chemistry are thus completely known, and the difficulty lies only in the fact that application of these laws leads to equations that are too complex to be solved. — Paul Dirac ABSTRACT The present work addresses challenges with the performance and pro- cessability of organic semiconductors for their application in organic field-effect transistors (OFET). One fundamental issue is the inevitable presence of series resistances across interfaces between conducting and semiconducting materials. It is demonstrated that a time consuming im- mersion step during fabrication of injection-promoting self-assembled monolayers (SAM) can be carried out as fast as five seconds with in- significant drawbacks to the performance, if processing parameters are optimized. A novel SAM-forming molecule, featuring remarkable perfor- mance and stability, is presented and characterized. It is subsequently used to demonstrate a concept for the fabrication of unipolar n- and p-type OFETs from a single semiconductor, determined selectively by the presence or absence of an injection layer treatment. Also, the electri- cal decoupling of typical charge immobilization sites, provided by inor- ganic dielectric materials, from the delicate transistor channel via a thin polymer coating is investigated. Furthermore, an approach to prevent in- terlayer mixing in solution processing of multilayer stacks is presented, using novel polymers with solubility providing alkyl-chains, which can be detached and render the film insoluble by a thermal stimulus after deposition. The results that are presented in this thesis show that combining ad- vances in chemical material design and interface tailoring with advances in deposition and processing techniques, in order to unfold the potential of organic electronics, is a matter of engineering. ZUSAMMENFASSUNG Diese Arbeit befasst sich mit den Herausforderungen an Leistungsfä- higkeit und Prozessierbarkeit von organischen Halbleitern für die An- wendung in organischen Feld-Effekt Transistoren (OFET). Ein grundle- gendes Problem stellen hierbei unvermeidliche elektrische Widerstän- de an Kontakten zwischen leitenden und halbleitenden Materialien dar. Es wird gezeigt, dass ein zeitaufwändiger Herstellungsschritt bei der vii Tauchbeschichtung von injektionsbefördernden selbst-organisierten Mo- nologen (SAM) mittels optimierter Prozess-Parameter bis zu einer Dau- er von fünf Sekunden beschleunigt werden kann. Ein neuartiges Mo- lekül zur Herstellung von SAMs, welches herausragende leistungsför- dernde Eigenschaften und eine hohe Stabilität aufweist, wird vorgestellt und charakterisiert. Diese SAMs werden anschließend verwendet um ein Konzept zur Herstellung von selektiv unipolaren n- und p-typ Transis- toren aus dem selben Halbleiter, differenziert durch An- oder Abwesen- heit der Injektionsschicht, zu demonstrieren. Außerdem wird das Ent- koppeln von elektrischen Fallenzuständen, welche an Grenzflächen zwi- schen organischen Halbleitern und anorganischen Dielektrika entstehen, vom störungsanfälligen Transistorkanal mittels einer dünnen Polymer- Beschichtung untersucht. Darüber hinaus wird ein Konzept vorgestellt das es erlaubt ein Durchmischen an der Grenzfläche zwischen aufeinan- derfolgend aus Lösung prozessierter Schichten zu verhindern. Hierfür werden neuartige halbleitende Polymere verwendet, deren löslichkeits- vermittelnde Seitenketten nach der Filmherstellung durch einen thermi- schen Reiz abgespalten werden können. Die Ergebnisse die in dieser Arbeit vorgestellt werden zeigen, dass die Zusammenführung von Entwicklungen im chemischen Design neu- er Materialien und Grenzflächenmodifizierungen mit Fortschritten in der Prozessierungstechnik eine entscheidende ingenieurwissenschaftli- che Herausforderung darstellt, um das Potential der organischen Elek- tronik auszuschöpfen. viii CONTENTS i introduction1 1motivation 3 1.1 Scope of this work . 4 1.2 Outline . 5 2 theoreticalfundamentals 7 2.1 Organic semiconductors . 7 2.1.1 Charge delocalization in organic matter . 8 2.1.2 Charge transport and doping . 9 2.2 Field Effect Transistors . 10 2.2.1 Working principle . 10 2.2.2 Modeling . 13 2.3 Self-assembled monolayers . 20 2.3.1 Structure and composition . 20 2.3.2 Accumulation, growth and assembly . 22 2.3.3 Application of SAMs for injection barrier reduction 24 3materialsandmethod 27 3.1 Sample preparation . 27 3.1.1 Thin film deposition . 27 3.1.2 OFET architectures . 31 3.1.3 Electrodes . 32 3.1.4 Semiconductors . 32 3.1.5 Gate dielectrics . 34 3.2 Characterization . 35 3.2.1 Electrical device characterization . 35 3.2.2 Thin film characterization . 39 3.3 Partially automatized evaluation with the “EVA” Origin script . 42 ii results and discussion 43 4 charge injection properties at the metal-semiconductor interface 45 4.1 SAM accumulation in printing-relevant timescales . 45 4.1.1 OFET optimization with PFDT . 48 4.1.2 Surface coverage, chemical surface composition and WF-shift via PES . 49 4.1.3 Angle determination via IRRAS . 50 4.1.4 Coverage, ordering and performance . 53 ix x contents 4.1.5 Influence of extensive ambient exposure . 54 4.2 Characterization of a novel high performance electron in- jection SAM . 55 4.2.1 Analytical characterization of Juls SAM . 56 4.2.2 OFET optimization with Juls . 56 4.2.3 Ambient processed OFETs with printed electrodes on flexible substrates . 59 4.2.4 Protection of silver electrodes in OFETs from degra- dation in ambient conditions . 61 4.3 Selective OFET operation polarity for complementary logic gate devices . 63 4.3.1 Bipolar- versus selective unipolar transport . 64 4.3.2 PEIE- and Juls-treated unipolar OFETs with PDTDPP- alt-BTZ . 66 5 passivation of trap states at organic/inorganic interfaces 69 5.1 Wetting and passivation behavior . 70 5.2 Successive I/V sweep bias stress effect . 72 5.3 Temperature behavior . 75 5.4 Influence of thermal treatment on the bias stress effect . 78 5.5 Charge trapping mechanisms in silicon oxide . 78 6 polymers with thermally stimulated solubility re- duction 81 6.1 Thersol 1-3: Analytic characterization and device perfor- mance . 82 6.1.1 Thersol 1 (P(HtHC-NDI-4HT2)) . 82 6.1.2 Thersol 2 (P(tHC-NDI-4HT2)) . 85 6.1.3 Thersol 3 (P(HtODC-NDI-T2)) . 87 6.2 Influence of pyrolysis and solvent washing . 87 6.2.1 Thickness reduction upon pyrolysis and solvent washing . 89 6.2.2 Channel resistance upon pyrolysis and solvent washing . 90 6.2.3 Morphology, p-delocalization and electrical per- formance . 92 iii conclusion 93 7 summaryandconcludingdiscussion 95 Appendix 101 bibliography 103 LISTOFFIGURES Figure 1 Charge delocalization via p-electron systems . 8 Figure 2 Cross section FET, linear and saturated regime . 11 Figure 3 Schematic illustration: OFET working principle 12 Figure 4 Ideal FET in MOFET . 16 Figure 5 Schematic illustration: TLM . 17 Figure 6 Example TLM corrected mobility . 18 Figure 7 SAM binding mechanism . 21 Figure 8 Island growth of SAMs . 23 Figure 9 Schematic illustration: injection barrier at the metal-semiconductor interface . 25 Figure 10 Schematic illustration: thermal evaporation . 28 Figure 11 Schematic illustration: spin coating . 29 Figure 12 Schematic illustration: chemical vapor deposition 30 Figure 13 OFET architectures . 31 Figure 14 Ag electrode oxidation . 33 Figure 15 Semiconductor polymers . 34 Figure 16 Dielectric polymers . 35 Figure 17 Output- and transfer I/V-characteristic . 38 Figure 18 Schematic illustration: Photoelectron Spectroscopy 40 Figure 19 Schematic illustration: Infrared Reflection Ab- sorption Spectroscopy . 41 Figure 20 Chemical structure PFDT and OFET stack . 46 Figure 21 OFET devices with PFDT . 47 Figure 22 XPS and UPS data PFDT . 49 Figure 23 IRRAS data PFDT . 51 Figure 24 Compilation data PFDT . 52 Figure 25 Oxygen contamination PFDT . 54 Figure 26 Chemical structure Juls . 55 Figure 27 PES data on Juls . 57 Figure 28 Wetting behavior of Juls . 58 Figure 29 OFETs with Juls SAM . 59 Figure 30 OFETs with Juls injection layer on PET substrate 60 Figure 31 Oxydation protection of Ag contacts with Juls SAM . 62 Figure 32 Effective gate voltage . 65 Figure 33 Off-state quenching in ambipolar FETs . 65 Figure 34 Selective unipolar OFETs with PDTDPP-alt-BTZ 66 xi xii List of Figures Figure 35 Selective unipolar OFETs with PDTDPP-alt-BTZ and Juls SAM . 67 Figure 36 Wetting envelope SiO2, ParyleneC, OTS . 70 Figure 37 BGBC SiO2